I think the amount of water and the force of the water we are seeing is probably rather low and something similar to capillary action is causing the small amount of water to dampen the surface, so the effects are going to be quite subtle. Or maybe the long term effects of this have just been overlooked in the past?

That's weird CRISM does not see anything. They argue that flows are much smaller than the CRISM pixel scale (18m).

There's a spectral processing technique called spectral mixture analysis (SMA) which has been used in many studies (including by yours truly) to determine the sub-pixel fractional area cover of different substances. If water covers a reasonable portion of the area of a pixel, SMA should be able to tease it out. I believe SMA has been used for Mars studies, but if not, it's a well known technique. If it's not been used in this case, this suggests that the CRISM investigators have used equally good spectral techniques for teasing out fractional coverage.

The lack of a water spectra suggests that (1) CRISM has been unlucky enough to look when water wasn't present, (2) there is so little water at any given time that it doesn't trigger a measurable spectral response, (3) water isn't involved.

One possibility for the source, if it is briny water, is overnight frost that is melted by contact with the salty bedrock, forming a brine. In that case, the time to try to "catch it in the act" might be the early morning hours.

There's also no reason why multiple CRISM images of these suspect locations cannot be stacked for improved resolution. Obviously the closer together in time the images in the sequence are captured, the better.

Mars Climate Sounder onboard MRO can detect water vapor (and thus you can calculate humidity as it detects temperature also).

But it probably wouldn't work for this scenario. It's not ideal for detecting these values in the lowest layers in the atmosphere and whatever water comes out of these features probably won't significantly effect the water vapor concentration at higher altitudes.

Unfortunately, the MCS water detection capability has been compromised by less-than-ideal behavior of the "in band" water channels (i don't remember their filter names). As such, its capability remains theoretical. Hopefully, EMCS on ExoMars-TGO will be able to avoid this problem.

CRISM measures an integrated water column on a regular basis (i.e., Smith et al., 2009; http://adsabs.harvard.edu/abs/2009JGRE..11400D03S). However, one would really want to get the sporadic limb "scans" by CRISM to get a better sense of humidity levels in the atmosphere. These have been analyzed (and still are) by Smith et al., but only published in abstracts at the moment. Also, analogous to the issue pointed out by Drskywxlt for thermal sounding, the lower scale height is difficult to resolve in a meaningful way for water by CRISM (radiation field is "thermalized" by multiple scattering in the CRISM regime).

The best hope for orbital, vertically-resolved water vapor measurements would be sub-mm/microwave sounding, which are basically insensitive to the aerosol components in the atmosphere, at least in terms of confounding water line analyses.

Amusingly, it's similar images of water that make me wonder if the albedo changes simply have to do with the clumpiness of the dust in the streaks. There's activity in snowpack called either sloughing/sluffing. Basically by altering only the texture of the snow, it creates streaks like the ones found in the dust on Mars:

I guess this sort of thing is more common with SAR... but you can see it in plain old visible light. The whole slope is bright white snow. Up close the color of the sluffs is no different. They're just reflecting the sky back to the camera differently.

Like snow, the dust probably falls in dry and sparsely packed, and clumps up more when something starts it rolling.

I wonder if the streaks are just clumpier dust on airfall dust. They do need a trigger, though. That could be a tiny bit of water starting a basically dry sluff.

I really like your photo of the snow sloughing features. I'm just starting a wikipedia article about martian dark slope streaks, and am wondering if I could use the photo there. If so, how should I credit it? Thanks.Tom

I really like your photo of the snow sloughing features. I'm just starting a wikipedia article about martian dark slope streaks, and am wondering if I could use the photo there. If so, how should I credit it? Thanks.

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